Typically, in a prechamber-type gas engine 1, among so-called gas engines, as illustrated in FIG. 6 to FIG. 8, since the main combustion chamber 4 for combusting premixed lean-burn gas is defined by a piston top surface portion at TDC (top dead center) of the piston 2 together with a lower surface of the cylinder head 3 and the cylinder liner, in order to inject a flame generated in the precombustion chamber 5 from the injection hole 6 of the precombustion chamber 5 to the main combustion chamber 4, it is necessary to allow the torch-like flame jet (torch jet, tj) from the injection hole 6 to reach a cylinder edge uniformly.
That is, in the prechamber-type gas engine 1 illustrated in FIG. 6 to FIG. 8, the precombustion chamber 5 is provided with a plurality of injection holes 6 which are in communication with a mixed gas-ignition device (not shown) such as an ignition plug or an igniting-fuel injection valve and with the main combustion chamber 4, and the injection holes 6 are arranged so that the extending directions thereof are spaced at the same angular interval θ in a planar view. For example, in the case of the number of the injection holes of n, the angular interval is represented by θ=360/n degrees, and in the case of FIG. 6, n=6 and θ=60 degrees.
Further, a fuel gas is supplied to the precombustion chamber 5 so as to form a rich mixture which is likely to be ignited upon compression by the piston 2. A lean mixture is introduced into the main combustion chamber 4 through a supply valve 7. Further, the mixture gas is combusted in the main combustion chamber 4 to form exhaust gas, which is discharged through an exhaust valve 8.
In a gas engine comprising the above structure, the rich mixture in the precombustion chamber 5 is ignited by the ignition device to be combusted, and it is injected into the main combustion chamber 4 to form a torch jet tj, and then to ignite the lean mixture. After combustion in the precombustion chamber 5, the flame in the main combustion chamber 4 proceeds along the radial direction from the torch jet tj and propagates along the circumferential direction to expand as indicated by the dashed line of FIG. 6. Then, due at least in part, to the combustion in the main combustion chamber 4, the piston 2 is moved downward from TDC to rotate the crankshaft. The piston 2 comprising reached the bottom dead center (BDC) moves upward again, where the gas is discharged through the exhaust valve 8 in the exhaust stroke. During the series of strokes as described above, a region where the flame propagation is delayed may be formed between the injection directions of the torch jets tj. Therefore, unburnt gas in this region, for example knocking region K, may cause knocking, which is a factor that prevents improvement of thermal efficiency or output of the gas engine.
Patent Document 1 (identified below) discloses a combustion chamber of a large-sized gas engine. That is, Patent Document 1 discloses a combustion chamber applied to a gas engine, which comprises a piston comprising an upper surface of which circumferential edge portion comprises a flat surface and which comprises a protruded portion formed in the middle portion of the piston. According to Patent Document 1, the volume of periphery of the main combustion chamber is larger and the fuel becomes likely to flow to the periphery, and a portion such as a valve recess or a top clearance portion, which conventional main combustion chamber usually comprise, where the density of the gas tends to be relatively lower in the lean gas, becomes unnecessary, whereby lean gas which is to be ignited in a delayed fashion does not exist in the periphery.
Patent Document 2 (identified below) discloses a prechamber-type gas engine comprising a squish portion formed between an upper edge surface of a combustion piston at TDC and a lower surface of a cylinder cover, and the piston comprises a plate-like cavity used for a prechamber-type gas engine where a flame generated in the prechamber is injected from a prechamber hole of the prechamber to the combustion chamber, wherein the ratio of the parallel length T of the upper edge surface of the piston facing to the squish portion to the inner diameter B of the cylinder of the engine is set to be θ<T/B≤0.07. According to Patent Document 2, by maintaining the ratio within the range of θ<T/B≤0.07, even if the cylinder inner diameter excesses a certain value, a part of the flame jet injected from the prechamber to the combustion chamber reaches sufficiently deep into the squish portion. According to the document, the flame propagation of the flame jet thereby pushes out unburnt mixture gas remaining in the squish portion, and the amount of the unburnt mixture gas remaining in the squish portion may be reduced.
Patent Document 1: WO2011/080914
Patent Document 2: JP 2008-286143 A